(1) Field of the Invention
The present invention generally relates to a testing method of a magnetic recording medium and a measurement method of a time constant of remanence thermal decay in a magnetized sample, and more particularly to a testing method of a magnetic recording medium to evaluate a life of record on the magnetic recording medium with a predetermined recording density and a method for measuring a time constant of remanence thermal decay in a magnetized sample.
In recent years, the recording density on the hard disk unit is being increased at a rate of about 60% a year. It is expected that the recording density in the longitudinal recording will reach to the upper limit in the near future. This is based on the reason that the higher the recording density on the magnetic recording medium the easier the more likely the occurrence of superparamagnetization due to thermal fluctuation.
Thus, it is expected that a magnetic recording unit to which the magnetic recording medium is set is required to be given a guarantee for a life of recorded bits (referred to as a bit record life). A technique for evaluating the bit record life for the magnetic recording medium is important.
Thus, it is necessary to improve the technique for evaluating the bit record life and to establish a method for guaranteeing the bit record life for the hard disk unit.
(2) Description of the Related Art
In accordance with various detection method, a magnetic recording medium in which a remarkable relaxation of magnetization due to thermal fluctuation be recognized in a short evaluating period such as a few days or a few weeks can be easy to test. For example, there is a method for reproducing recorded contents at predetermined intervals from a hard disk in which a predetermined file is recorded and detecting omission of the recorded contents. There is also a method for evaluating a hard disk by detecting reproduction output at predetermined intervals or by observation of the magnetic recording medium using a magnetic force microscope after signals are recorded in the magnetic recording medium using a read-write tester.
In addition, as a testing method for a test body having a long record life, in general, there is an accelerated life test. In the accelerated life test, the change, with the passage of short time, of the state of the test body at elevated temperatures is measured. As a result, the change, with the passage of long time, of the state of the test body at the room temperature can be estimated in a short time.
However, since the magnetic recording medium is formed of a magnetic thin film, such an accelerated life test can not applied to the magnetic recording medium. That is, the product (Ku.times.Va) of an anisotropy energy constant (Ku) of each of magnetic particles forming the magnetic thin film and an activation volume (Va) which is a unit of the magnetization reversal is an increasing function (Ku.times.Va=F(T)) with respect to temperature T, so that the energy barrier could not lowered as expected. Thus, it is difficult to accelerate a shrinkage of record life of the magnetic recording medium by elevating the temperature, so that the accelerated life test can not be applied to magnetic recording medium.
Thus, in order to evaluate the magnetic recording medium having the long record life, responses to the thermal fluctuation should be measured for a long time, and the tendency of relaxation should be detected based on the characteristic of the measured responses. The record life can then be evaluated.
For example, a sample piece of the magnetic recording medium is applied with the saturation magnetization in a direction, and the time response of the remanence (residual magnetization) is then measured after the external magnetic field is removed. To measure the remanence, for example, a vibrating sample magnetometer (VSM) or a superconducting quantum interference device (SQUID) is used.
Although the vibrating sample magnetometer has a low measurement sensitivity, the vibrating sample magnetometer can measure the remanence of small samples piled up. From the view point of accurate measurement, it is preferable to use the superconducting quantum interference device for the measurement.
In the superconducting quantum interference device (SQUID), the measurement is performed under a condition that the sample is maintained at the room temperature. The applied magnetic field is removed and the residual magnetic moment is measured as a function of time. Usually, the measurement is continued for several hours or a few days, or at most about one month. The measurement data is used to extrapolate the amount of relaxation due to thermal fluctuation.
From many studies, it is known that the remanence is substantially logarithmically varied with the passage of time. Based on this magnetic relaxation characteristic, the record life can be relatively accurately evaluated.
The above mentioned conventional method is practical for the test of the magnetic recording medium. However, the conventional method can be used only for uniform magnetization. It is not clear what degree of high density the recording can be performed at. The method for measuring the variation of the remanence in uniform magnetized pattern with the passage of time can not be substituted for a method for evaluating the bit record life. This is because of the following reasons. First, the state of magnetic record in a bit pattern differs from the state of magnetic record in a uniform magnetic pattern. Second, in the bit pattern, the demagnetizing field is strong so that the bit magnetization may be easily reversed. Thus, the magnetic relaxation of the bit pattern is greater than that of the uniform magnetizing pattern.
A simulation experiment based on a virtual magnetic particle structure which is suitable for calculation has been proposed by S. H. Charap et al. However, simulation experiments so far can not correctly reflect the characteristic of an actual medium. In addition, the theoretical calculation based on an actual structure of a medium has not yet sufficiently progressed.
That is, the conventional testing method for the magnetic recording medium has the following disadvantages.
First, a bit pattern suitable for the evaluation of the bit record life in a case where bits are magnetically recorded at a linear density at which the performance should be guaranteed has not yet been known. That is, the bit pattern includes two types of magnetizing directions. Although there are magnetized bits having two magnetizing directions microscopically, a difference between the total amount of magnetization in one of the directions and the total amount of magnetization in the other one of the directions appears microscopically. This fact is not considered in the conventional evaluation method.
Second, although it is known that the time response characteristic regarding the magnetic relaxation due to the thermal fluctuation is represented by a monotonously decreasing curve, the sequential variation of the magnetization under a condition in which the characteristic regarding the relaxation time is changed (the rate of the variation of the remanence with the passage of time) can not be processed in accordance with the conventional method. That is, in a case where the material characteristic parameters peculiar to material and a structure are extracted and the bit record life is obtained considering a state where the phenomenon of the magnetic relaxation followed by the statistical fluctuation in principle be transiently varied, a new technology is required.
Third, in a case where the magnetic relaxation of a bit pattern due to the thermal fluctuation is measured, a successful method for evaluating characteristic with the arrangement of bit pattern has not yet been obtained at all. A method for selecting information items provided with measurement points which are measured in accordance with the bit arrangement and extracting contents regarding the type of bit from the measurement data is needed. However, there is not such a method.
Fourth, a technique which relates the rate variation of the remanence in the bit cell with the passage of time to the bit record life has not yet been proposed. In this technique, the bit record life is obtained by using the fluctuation characteristic, obtained as the result of the measurement of the uniform magnetization, logarithmically varying with the passage of time (the conventional technique). This technique is needed for the evaluation of the bit record life.